This application claims the priority benefit of Taiwan application serial no. 90112761, filed May 28, 2001.
1. Field of Invention
This invention relates to a fabrication method for a read-only memory device. More particularly, the present invention relates to a fabrication method for an oxide layer-nitride layer-oxide layer (ONO) structure and a field oxide layer of a flash memory device.
2. Description of Related Art
Flash memory device is an electrically-erasable-programmable-read-only memory device, which has the advantages of being programmable, erasable and the ability of retaining data when the power is interrupted. A flash memory device is thus commonly used in personal computer and electronic devices. A flash memory device is also a non-volatile type of memory (NVM), which has the advantages of being small in dimension, speedy in in data retrieval and storage, and low power consumption. Furthermore, the erasure of data in a flash memory device uses the xe2x80x9cblock-by-blockxe2x80x9d method; therefore, it also has the advantage of having a faster operational speed.
For a typical flash memory device, the floating gate and the control gate are formed with doped polysilicon. Electrons are injected into the floating gate and evenly distributed in the polysilicon layer of the floating gate during programming. However, if defects are present in the tunnel oxide layer underneath the polysilicon floating gate layer, current leakage is easily resulted to adversely affect the reliability of the device.
The current technical development on flash memory devices includes forming the floating gate with silicon nitride. The control gate, however, is still formed with polysilicon. When a voltage is applied to the control gate and the source region of this type of device to perform the programming function, electrons in the channel region near the vicinity of the drain region are injected into the silicon nitride floating gate. Since silicon nitride has the characteristic of being able to capture electrons, the electrons injected into the silicon nitride floating gate are not evenly distributed in the entire floating gate. Instead, the electrons are Gaussian distributed and concentrated locally in the floating gate. Because the electrons are concentrated locally in the floating gate, the sensitivity to any defect in the tunnel oxide layer is minimized. As a result, the occurrence of a current leakage is mitigated.
FIGS. 1A through 1D are schematic, cross-sectional views, illustrating successive steps of fabricating a silicon nitride floating gate for a read-only memory according to the prior art.
Referring to FIGS. 1A and 1B, a pad oxide layer 102 and a silicon nitride layer 104 are sequentially formed on a provided substrate 100. The silicon nitride layer 104 and the pad oxide layer 102 are then patterned to expose a portion of the substrate 100. Thereafter, thermal oxidation is conducted to form a field oxide layer 106 on the exposed substrate 100. The silicon nitride layer 104 and the pad oxide layer 102 are further removed, leaving the field oxide layer 106 on the substrate 100.
Referring to FIGS. 1C and 1D, an oxide layer 108 and a silicon nitride layer 110 are sequentially formed on the substrate 100. A wet oxidation process is then conducted to form an oxide layer 112. The oxide layer 112, the silicon nitride layer 110 and the oxide layer 108 are further patterned to form an oxide layer-nitride layer-oxide layer (ONO) structure of a tunnel oxide layer 108a, a silicon nitride floating gate layer 110a and an oxide dielectric layer 112a. Thereafter, ions are implanted between the ONO structures to form the buried bit line in the substrate 100. A polysilicon control gate structure layer is also formed on the silicon oxide dielectric layer 112a. According to the conventional approach in fabricating a silicon nitride read-only memory, the silicon oxide dielectric layer 112a is formed subsequent to the formation of the field oxide layer 106. The manufacturing process is thus more time consuming, complicated and costly.
The invention provides a fabrication method for a silicon oxide layer-silicon nitride layer-silicon oxide layer structure and a field oxide layer of a flash memory device, to simply the overall manufacturing process.
The present invention provides a combined fabrication of a silicon oxide layer-silicon nitride layer-silicon oxide layer structure and a field oxide layer, to greatly reduce the processing steps and cost
The invention provides a fabrication method for a silicon nitride read-only memory, wherein a first oxide layer and a silicon nitride layer are sequentially formed on a provided substrate. The first oxide layer and the silicon nitride layer are then patterned to form an opening, exposing a part of the substrate. Thereafter, an oxidation process is conducted to form a second oxide layer on the silicon nitride layer, wherein concurrently, a field oxide layer is formed on the exposed part of the substrate. The second oxide layer, the silicon nitride layer and the first oxide layer are then patterned to form an oxide dielectric layer, a silicon nitride floating gate layer and an oxide tunnel layer.
According to the present invention, a combined manufacturing of the oxide dielectric layer and the field oxide layer is provided to greatly reduce the number of manufacturing steps and simply the manufacturing process.
According to the present invention, a combined manufacturing of the oxide dielectric layer and the field oxide layer is provided to greatly reduce the manufacturing cost.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.